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	<title>构造函数</title>
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	构造函数的内部原理:
	new
	会让函数头部隐式的加上this = {} --- (var this = {})

	执行this.xxx = XXX;

	隐式的返回this(尾部retuen this;)


	<script>
	function Student (name,age,sex){
		// var this = {
		// 	name  : ''
		// 	age :
		// };
		this.name = name;
		this.age = age;
		this.grade = 2017;
		//return this;
	}

	var student = new Student ('zhanghua','18','male'); 
	</script>
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